2D graphics are used to overlay video frames with primitives such as text. For example, a news program might include a video of a news presenter overlayed with a news crawl scrolling across the top or bottom of the screen.
In an implementation, a graphics system processes an original video to superimpose the text thereon. The original video is initially stored in a video buffer. The system then draws the text on an overlay having the same size as the video buffer. The system processes the text and the video to output the video with the superimposed news crawl.
In one display mechanism, the entire display areas of the overlay and the video buffers are processed at this time. Further, another display mechanism uses a macro block-based technique in which, after an entire graphic primitive object is drawn, a marking is done to indicate the macro blocks to be processed. The marking is performed by first determining the minimum rectangle that covers the entirety of the primitive. The mechanism then marks all the macro blocks that are part of the minimum rectangle. Thus, if a line is drawn from the top left of a display screen to the bottom right of the display screen, the mechanism will mark all of the macro blocks because the minimum rectangle covering the line constitutes the entire display screen. Later, during a display function of this mechanism, the marked macro blocks are processed.